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#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

int msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);

Description

    long mtype;    /* message type */
    char mtext[];  /* message text */

mtype is a positive integer that can be used by the receiving process for message selection. mtext is any text of length msgsz bytes. msgsz can range from 0 to a system imposed maximum.

msgflg specifies the action to be taken if one or more of the following are true:

The number of bytes already on the queue is equal to

msg_qbytes (see intro(2) ).

The total number of messages on all queues system-wide is equal to the

system-imposed limit.

These actions are as follows:

If

(msgflg&IPC_NOWAIT) is true, the message is not sent and the calling process returns immediately.

If

(msgflg&IPC_NOWAIT) is false, the calling process suspends execution until one of the following occurs:

The condition responsible for the suspension no longer exists, in which case the message is sent.

msqid is removed from the system (see msgctl(2) ). When this occurs, errno is set to EIDRM, and a value of -1 is returned.

The calling process receives a signal that is to be caught. In this case the message is not sent and the calling process resumes execution in the manner prescribed in signal(3C) .

msgrcv() reads a message from the queue associated with the message queue identifier specified by msqid and places it in the user defined structure pointed to by msgp. The structure must contain a message type field followed by the area for the message text (see the structure mymsg above). mtype is the received message’s type as specified by the sending process. mtext is the text of the message. msgsz specifies the size in bytes of mtext.

The received message is truncated to msgsz bytes if it is larger than msgsz and (msgflg&MSG_NOERROR) is true. The truncated part of the message is lost and no indication of the truncation is given to the calling process.

msgtyp specifies the type of message requested as follows:

If

msgtyp is 0, the first message on the queue is received.

If

msgtyp is greater than 0, the first message of type msgtyp is received.

If

msgtyp is less than 0, the first message of the lowest type that is less than or equal to the absolute value of msgtyp is received.

msgflg specifies the action to be taken if a message of the desired type is not on the queue. These are as follows:

If

(msgflg&IPC_NOWAIT) is true, the calling process returns immediately with a return value of -1 and sets errno to ENOMSG.

If

(msgflg&IPC_NOWAIT) is false, the calling process suspends execution until one of the following occurs:

A message of the desired type is placed on the queue.

msqid is removed from the system. When this occurs, errno is set to EIDRM, and a value of -1 is returned.

The calling process receives a signal that is to be caught. In this case a message is not received and the calling process resumes execution in the manner prescribed in signal(3C) .

Return Values

If msgsnd() or msgrcv() return due to the receipt of a signal, a value of -1 is returned to the calling process and errno is set to EINTR. If they return due to removal of msqid from the system, a value of -1 is returned and errno is set to EIDRM.

Upon successful completion, the return value is as follows:

msgsnd()

returns a value of 0.

msgrcv

returns the number of bytes actually placed into mtext.

Otherwise, a value of -1 is returned and errno is set to indicate the error.

Errors

msgsnd() fails and sends no message if one or more of the following are true:

EACCES

Operation permission is denied to the calling process (see intro(2) ).

EAGAIN

The message cannot be sent for one of the reasons cited above and (msgflg&IPC_NOWAIT) is true.

EFAULT

msgp points to an illegal address.

EINVAL

msqid is not a valid message queue identifier.

EINVAL

mtype is less than 1.

EINVAL

msgsz is less than zero or greater than the system-imposed limit.

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro(2) ).

msg_qnum

is incremented by 1.

msg_lspid

is set to the process ID of the calling process.

msg_stime

is set to the current time.

msgrcv() fails and receives no message if one or more of the following are true:

E2BIG

The length of mtext is greater than msgsz and (msgflg&MSG_NOERROR) is false.

EACCES

Operation permission is denied to the calling process.

EFAULT

msgp points to an illegal address.

EINVAL

msqid is not a valid message queue identifier.

EINVAL

msgsz is less than 0.

ENOMSG

The queue does not contain a message of the desired type and (msgtyp&IPC_NOWAIT) is true.

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid (see intro(2) ).

msg_qnum

is decremented by 1.

msg_lrpid

is set to the process ID of the calling process.

msg_rtime

is set to the current time.

See Also

intro(2) , msgctl(2) , msgget(2) , signal(3C)

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• Name
• Synopsis
• Description
• Return Values
• Errors
• See Also